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                        Introduction                                                                                                                                                    Chordata


Invertebrate Zoology

Kingdom:  Animalia, Phylum: Hemichordata



Phylum:  Hemichordata --

     Wormlike marine animals,



  Class:  Enteropneusta

        Family:   Harrimaniidae
        Family:   Protoglossidae
        Family:   Ptychoderidae
        Family:   Saxipendiidae

        Family:   Spengelidae

        Family:   Torquaratoridae

Class:  Graptolithina (extinct)

Class:  Pterobranchia

Class:  Planctosphaeroidea

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Invertebrate Classification


          The Hemichordata is a phylum of worm shaped marine animals that are often thought of as a group closely related to the Echinodermata.  They were present in the Lower or Mid-Cambrian period so that there many fossils called graptolites.


           Their bodies are divided into three parts:  (1) a proboscis, (2) collar and (3) trunk.  They possess and open circulatory system and a complete digestive tract.  The gut does not have well developed muscles and food is transported primarily through it by cilia that cover the inside surface.


          A stomochord or diverticulum of the foregut occurs that is probably related to the notochord of the Chordata.  However, this may be the result of convergent evolution rather than homology.  A hollow neural tube is found in some species, which is thought to be a primitive trait that they share with the common ancestor of the Chordata.


General Characteristics of Hemichordata


          All members of the Hemichordata: Enteropneusta, or acorn worms, inhabit the marine environment.  They are wormlike, burrowing and range in size up to one meter.  A type animal is Balanoglossus.



          Body Regions. -- There are three:  a proboscis that resembles an acorn, a collar and a trunk.


          Gill Slits. -- These occur along the sides of the trunk.  They are closely allied with the Chordata.


          Body Plan. -- The mouth is located between the proboscis and collar.  The pharynx has a diverticulum.  The gill slits open through the body wall and to the outside of the animal.  A diverticulum runs from the pharynx, which has been homologized with a notochord.


          Nerve Cords. -- There is a dorsal nerve cord, which may be hollow in a few forms.  There is also a ventral nerve cord.


          Embryology. -- The egg undergoes cleavage similar to the starfish and Amphioxis.  They have a Tornaria larva, which resembles the Auricularia larva of the Holothuroidea (Echinodermata).  This suggests an echinoderm and chordate relationship.


          Economic Importance. -- There is no economic importance, however the animals give off an iodoform odor.







            Introduction                                                                                                                                                    Chordata